During supersonic flight, an airfoil surface of an aircraft is subjected to high temperatures resulting from frictional contact of the air against the airfoil surface. These high temperatures may change the properties of the material forming the airfoil surface so that the material is weakened. Also, these high temperatures may cause weakening of the structural members which support the airfoil surface.
In addition to changes in the properties of the materials forming the airfoil surface and the structural members which support the airfoil surface, high temperatures can produce large thermal stresses in the airfoil surface and in the structural supporting members for the surface. Consider, for example, an aircraft which may depart from an airstrip in the Arctic where the airfoil surface and its supporting members have an initial temperature of 0.degree. F. or less. After attaining supersonic speeds, the airfoil surface may reach a temperature of 1,000.degree. F. or more. Assuming rigid points of connection between the members making up the airfoil surface and points of rigid connection between the airfoil surface and the underlying support structure, large thermal stresses are set up during heating of the airfoil surface from 0.degree. F. to 1,000.degree. F.
With a large increase in its temperature, the airfoil surface would like the expand freely; however, it is prevented from doing so by the various connection points. Thus, there are resistance forces at the connection points which oppose expansion of the airfoil surface. To relieve the internal stresses in the airfoil surface, the airfoil surface will have a tendency to distort which can alter the aerodynamic properties of the airfoil surface. In an extreme case, distortion of the airfoil surface due to thermal stresses may cause rupture of the surface or metal failure at one or more of the connecting points between the components of the airfoil surface or between the airfoil surfaces and the underlying structural support.
In view of the above problems which result from temperature variations in an airfoil surface, it would be desirable to have an airfoil structure which can absorb thermal stresses with the airfoil surface then being partially isolated from its supporting structure. By thermally isolating the airfoil surface from the supporting structure, the high temperatures developed by the airfoil surface would not be transmitted to the underlying structure. Thus, the supporting structure would not be weakened by being subjected to high temperatures. The airfoil surface could then be formed of a temperature-resistant material, such as stainless steel, while the underlying support structure could be formed of a more conventional material such as aluminum.
Further, in isolating the air foil surface from the underlying support structure, it would be desirable to provide an airfoil surface formed of a skin construction that is highly resistant to thermal stresses. Such a skin construction would be light in weight and yet capable of undergoing extreme fluctuations in temperature, such as a fluctuation of 2,000.degree. F. or more, without rupture and without imposing unacceptably high stresses on the underlying support structure. Such a skin construction could be employed, not only for airfoil surfaces for conventional aircraft, but also for airfoil surfaces for a space reentry vehicle which would be exposed to even greater temperature fluctuations.
Such a skin construction could also be used for applications that are unrelated to airfoil surfaces but in which great temperature fluctuations are experienced. For example, in transporting high temperatures fluids through a conduit, the inner conduit surface may be formed of a skin construction which is thermally isolated from the outer portion of the conduit that serves as a supporting structure for the inner skin. The inner skin of the conduit would absorb thermal stresses so as to preserve the integrity of the skin and to isolate the thermal stresses from the outer portion of the conduit. In this manner, the conduit could be constructed more cheaply by forming the inne skin surface of a high temperature material such as stainless steel while forming the outer portion of the conduit of conventional materials such as cast iron, steel, aluminum, or elastomers that are serviceable at reduced temperatures.